The present invention relates to flame retardant polyolefin resin pre-expanded particles suitable for the production of thermal insulation materials, cushioning packaging materials, returnable delivery boxes, automobile bumper core, electrical and electronic parts and so on, particularly electrical and electronic parts, and to in-mold foamed articles prepared by in-mold foaming of the pre-expanded particles.
Polypropylene in-mold foaming products are superior in chemical resistance, heat resistance, impact resistance and distortion restoration rate after compression as compared to polystyrene in-mold foaming products. Thus they have been widely used as cushioning packaging materials, returnable delivery boxes, automobile parts such as bumper core, side impact energy absorber and floor material, and others.
In general, however, cellular molded articles of polyolefin resins have the drawback of being flammable, while they have advantages as mentioned above. In particular, cellular molded articles have a higher flammability as compared with non-cellular molded articles and easily burn.
In recent years, automobile parts, building materials, and electrical and electronic parts are strongly demanded to have flame resistance or self-extinguishing property. In order to meet the demand, investigation has been made about production of cellular molded articles imparted with flame resistance.
As a method for imparting a flame resistance to foamed articles (polyolefin foamed articles prepared by an atmospheric pressure foaming method or a pressure foaming method), there is known a method wherein a resin is incorporated with a flame retardant such as an inorganic flame retardant, a halogen-containing flame retardant or a phosphorus-based flame retardant, melt-kneaded at a temperature lower than the decomposition temperature of a thermally decomposable blowing agent and molded into a desired shape such as sheet, and the molded article is expanded by heating at a temperature higher than the melting point (softening point) of the resin and the decomposition temperature of the blowing agent.
For example, JP-A-3-269029 discloses production of flame retardant foamed articles having excellent flexibility, heat resistance, mechanical property, heat insulation property and electric property together with a high flame resistance by molding a flame retardant expandable composition comprising 100 parts by weight of a polyolefin resin, 30 to 200 parts by weight of an inorganic flame retardant such as magnesium hydroxide, 0.1 to 15 parts by weight of a blowing agent and at most 10 parts by weight of a crosslinking agent at a temperature lower than the decomposition temperature of the blowing agent, and heating the resulting molded article under pressure at a temperature higher than the melting point (softening point) of the resin and the decomposition temperature of the blowing agent to expand the molded article at an expansion ratio of 1.1 to 25.
Also, JP-A-5-331310 discloses production of flame retardant foamed articles without problems of generation of harmful gas and discoloration by, after subjecting to molding into sheets and crosslinking by means of electron beam, thermally expanding an expandable polyolefin resin composition comprising 100 parts by weight of a polyolefin resin, 1 to 40 parts by weight of a thermally decomposable blowing agent, 5 to 50 parts by weight of ammonium polyphosphate and 0.5 to 5 parts by weight of zinc stearate.
Further, JP-A-7-238178 discloses production of flame retardant foamed articles having high mechanical strength and good appearance by, after subjecting to molding into sheets and crosslinking by means of electron beam, thermally expanding a flame retardant expandable polyolefin resin composition comprising 100 parts by weight of a polyolefin resin, 5 to 200 parts by weight of a polysiloxane compound, and a blowing agent such as a thermally decomposable blowing agent or a liquefied gas blowing agent.
In JP-A-3-269029, JP-A-5-331310 and JP-A-7-238178, non-halogen flame retardants are used to provide flame retardant polyolefin foamed articles. However, since foamed articles are prepared by heating molded articles under atmospheric pressure or under pressure to decompose a decomposable blowing agent or to volatilize a volatile blowing agent to thereby foam the molded articles, the methods disclosed therein are difficult to apply to in-mold foaming wherein pre-expanded particles are filled in a mold and are fused together by heating with steam. That is to say, it is difficult to achieve good melt adhesion of the pre-expanded particles and good surface appearance by these methods. Further, since a polyolefin is crosslinked, these methods are disadvantageous from the viewpoints of cost and recycling.
It is known that additives incorporated into resins exert an influence on the cell structure of the obtained pre-expanded particles. If inorganic non-halogen flame retardants are incorporated into pre-expanded particles, a problem such as formation of combined cells or fine cells may arise, thus resulting in deterioration of moldability of the pre-expanded particles.
On the other hand, the following methods are known as a method for imparting a flame resistance to pre-expanded particles and in-mold foamed articles prepared therefrom.
For example, JP-A-4-363341 discloses a self-extinguishing foamed article wherein polyolefin expanded particles are fused together and a thermally expandable graphite powder which serves as a flame retardant are present at the adhesion interface between the expanded particles in an amount of 8 to 20% by weight based on the weight of the foamed article. This method requires a step for previously adhering the graphite powder onto the surface of the expanded particles prior to molding the particles in a mold, thereby making the graphite powder intervene at the interface between adjacent particles fused together. Therefore, not only the method is complicated, but also a problem arises that the melt adhesion of expanded particles deteriorates due to the presence of graphite powder in the surface of pre-expanded particles.
Further, JP-A-6-192465 discloses an in-mold foamed article prepared by wetting polyolefin expanded particles with an organic liquid, adding antimony trioxide and a flame retardant such as a brominated aromatic compound together with a film forming agent to the particles, mixing and sintering (fusing together) them in a conventional manner. It is disclosed that the amount of the flame retardant is from 4 to 10% by weight based on the total weight of the mixture. Working examples thereof show production of molded articles meeting a flame resistance of UL94HF-1.
JP-A-7-309967 discloses flame retardant polyolefin expanded particles which contain a bis(alkyl ether)tetrabromobisphenol A flame retardant and/or a bis(alkyl ether)tetrabromobisphenol S flame retardant and a flame retardant synergist such as antimony oxide, and which have excellent melt adhesion of particles in molding, secondary expandability and moldability as well as flame resistance (self-extinguishing property). Working examples thereof show production of molded articles having a flame retardant content of 1.7 to 5% by weight, an expansion ratio of 28 to 32 and an oxygen index of 27 to 32.
JP-A-10-147661 discloses flame retardant polyolefin pre-expanded particles containing ethylenebispentabromodiphenyl or ethylenebistetrabromophthalimide as a flame retardant and antimony oxide as a flame retardant synergist. It is disclosed that the amount of the flame retardant is from 1 to 30 parts by weight per 100 parts by weight of a polyolefin resin. Working examples thereof demonstrate production of molded articles meeting a flame resistance of UL94HF-2.
JP-A-9-227711 discloses flame retardant polyolefin expanded particles containing 1 to 20% by weight of a pentavalent phosphate compound having a brominated aromatic group or a brominated aliphatic group as a flame retardant.
However, halogen-containing flame retardants such as bromine-containing flame retardants raise a problem of generating a harmful gas such as a halogenated gas at the time of burning. At present, safety of flame retardants attracts attention from the viewpoint of environmental problem, and there is a movement to regulate the use of compounds which have a possibility of generating a harmful gas such as dioxin at the time of burning. For example, xe2x80x9cblue angel markxe2x80x9d which is an ecolabel in Germany regulates the use of organic halogen-based flame retardants containing chlorine or bromine. xe2x80x9cTCO95xe2x80x9d which is an ecolabel in Sweden prohibits the use of organic chlorine-containing compounds and organic bromine-containing compounds with respect to plastic parts having a weight exceeding 25 g intended to use in electrical and electronic products.
Thus, flame retardation system using no bromine-containing flame retardant or the like has been desired.
JP-A-11-315067 discloses compounds of the formula:
R7NHxe2x80x94(CH2)3xe2x80x94NR7xe2x80x94CH2CH2xe2x80x94NR7xe2x80x94(CH2)3xe2x80x94NHR7
wherein R7 is a group of the formula: 
which are useful as a stabilizer for organic materials such as polyolefins. It is disclosed that these compounds are also useful as a flame retardant for organic materials such as polyolefins. However, there is no disclosure concerning application of these compounds to polyolefin pre-expanded particles and in-mold foamed articles made thereof. In-mold foaming is carried out by filling polyolefin pre-expanded particles in a mold and heating them with steam or the like to fuse them together, thereby giving polyolefin in-mold foamed articles having a desired shape. Therefore, if moldability of pre-expanded particles is poor, commercial value of the products is remarkably lowered. It is known that additives incorporated into polyolefin resins exert an influence on cell formation, e.g., shape and size of cells and combination of cells, and they may cause deterioration of in-mold moldability and mechanical property owing to formation of combined cells or fine cells, generation of color shading and hindrance to melt adhesion of pre-expanded particles. JP-A-11-315067 does not disclose any information required for the application to polyolefin pre-expanded particles and in-mold foamed articles made thereof.
It is an object of the present invention to provide flame retardant polyolefin in-mold foamed articles which can be prepared with good moldability and which do no generate harmful gas when burning.
A further object of the present invention is to provide flame retardant polyolefin in-mold foamed articles having an excellent flame resistance and physical properties comparable to non-flame-retarded polyolefin in-mold foamed articles without using halogen-containing flame retardants.
Another object of the present invention is to provide flame retardant polyolefin pre-expanded particles without using halogen-containing flame retardants, which can be molded in a mold with good moldability to give in-mold foamed articles having excellent flame resistance and physical properties.
These and other objects of the present invention will become apparent from the description hereinafter.
In accordance with the present invention, there is provided a flame retardant polyolefin pre-expanded particle made of a resin composition comprising a polyolefin resin and a sterically hindered amine ether flame retardant.
Preferably, the sterically hindered amine ether flame retardant is a compound of the formula (1):
R1NHCH2CH2CH2NR2CH2CH2NR3CH2CH2CH2NHR4xe2x80x83xe2x80x83(1)
wherein R1 and R2 are an s-triazine moiety T of the formula (2): 
in which R1 is an alkyl group having 1 to 12 carbon atoms and R6 is methyl group, cyclohexyl group or octyl group; and either one of R3 and R4 is the s-triazine moiety T represented by the formula (2) and the other is hydrogen atom.
Preferably, the flame retardant polyolefin pre-expanded particle is such that it shows two fusion peaks on a DSC curve when measured by differential scanning calorimetry and the heat of fusion QH of the peak appearing on the higher temperature side is from 1.5 to 25.0 J/g.
The present invention also provides a flame retardant in-mold foamed article prepared by in-mold foaming of the flame retardant polyolefin pre-expanded particles mentioned above.
According to the present invention, problems as encountered when using halogen-containing flame retardants, e.g., generation of toxic gas, do not arise, since a non-halogen specific sterically hindered amine ether compound is used as a flame retardant for polyolefin resins. Also, problems as encountered when using inorganic non-halogen flame retardants, e.g., deterioration of in-mold moldability of pre-expanded particles owing to formation of combined cells or fine cells, do not arise. A good flame resistance can be imparted to in-mold foamed articles, which are prepared by filling pre-expanded particles in a mold followed by heating with steam to melt-adhere them together, without impairing the melt adhesion of pre-expanded particles and the surface appearance of the foamed articles. Further, since the foamed articles are prepared by in-mold foaming of pre-expanded particles, it is possible to obtain foamed articles having a complicated shape.